x86/HVM: clear upper halves of GPRs upon entry from 32-bit code

x86/HVM: clear upper halves of GPRs upon entry from 32-bit code

Hypercalls in particular can be the subject of continuations, and logic
there checks updated state against incoming register values. If the
guest manufactured a suitable argument register with a non-zero upper
half before entering compatibility mode and issuing a hypercall from
there, checks in hypercall_xlat_continuation() might trip.

Since for HVM we want to also be sure to not hit a corner case in the
emulator, initiate the clipping right from the top of
{svm,vmx}_vmexit_handler(). Also rename the invoked function, as it no
longer does only invalidation of fields.

Note that architecturally the upper halves of registers are undefined
after a switch between compatibility and 64-bit mode (either direction).
Hence once having entered compatibility mode, the guest can't assume
the upper half of any register to retain its value.

This is part of XSA-454 / CVE-2023-46842.

Fixes: b8a7efe8528a ("Enable compatibility mode operation for HYPERVISOR_memory_op")
Reported-by: Manuel Andreas <manuel.andreas@tum.de>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>